(Case No. 7033) ~1~99~8 FREQUE~CY MODULATED RAILROAD TE~ACK CIRCUIT
BACKGROIJND OF THE INVENTION
My invention pertains to a frequency modulated railroad track circuit. More particularly, the invention pertains to an audio frequency track circuit in which an audio frequency signal is modulated by frequency shift keying to provide a modulated track signal whose accurate reception may be veri-fied at the receiver to increase the safety of the track circuit operation.
All railroad track circuits, and particularly audio frequency overlay circuits, are subject to extraneous noise signals induced from sources other than and external to the track circuit source. This noise may interfere with the operation of the track circuit or even cause the registration of incorrect indications as to track occupancy conditions.
Such interference i8 particularly prevalent in electrified railroads where propulsion current flows in the rails. ~u~
e~ret~ Y~ e~ modulation (FM) techniques to modulate the transmitted track circuit signals in order to provide greater distinctiveness from the noise signals for the track circuit receiver have been used, frequently in the form of Frequency Shift Keying (FSK). However, frequency modulation itself is not the complete answer to improved reliability and safety.
The conventional FM receiver apparatus used may not detect the absence of one of the side band frequencies due to an extremely low level of the received track signal resulting from an apparatus fault or a broken rail. In addition, the ~149~i8 external noise may match one of the side band frequencies, although at a lower level. Therefore, a track circuit receiver arrangement is desirable which will check that the signal received from the rails has the proper characteristics and ``
conditions. For example, one of three very desirable checks is to determine that both side band frequencies are present and alternating at the initial modulation frequency or code rate. Further, the received signal should have the proper level determined in accordance with predetermined track section attenuation characteristics and, even if it has these proper characteristics, should also have a time duration of at least a predetermined minimum length which will eliminate very short induced pulses which may have equivalent charac-teristics.
Accordingly, an object of the invention is a frequency modulated railroad track circuit arrangement which verifies the reception of a proper frequency modulated track signal of predetermined sufficient signal level at the receiver end of the section prlor to enabling the registry of an unoccupied track section indication.
Another object of the invention is receiver apparatus, for a frequency modulated audio frequency railroad track circuit, which includes verification means to aseure that the received track signal correctly contains both side band frequencies of the central carrier signal and that the received signal is of at least a preselected minimum level and duration.
., -~149918 Still another object of the invention is a frequency shift keyed overlay track circuit which includes verification networks at the receiver location, to check the reception of a track signal having identical frequency characteristics to that transmitted and a preselected level in accordance with the known attenuation characteristics of the track section prior to enabling the registry of an unoccupied section indi-cation.
Also an object of the invention is a receiver arrangement, for a frequency modulated track circuit, which includes a first verification network, responsive only to a received track signal containing both side frequencies of a frequency shift keyed carrier signal and which occur at the correct modulation rate, and a second verification network, respon-sive only to a received track signal with at least a predeter-mined minimum level, and which registers an unoccupied track section indication only when both networks re~pond to the received signal for a predetermined minimum time duration.
Yet another object of the invention is an alternating current railroad track circuit arrangement in whieh a fre-quency shift keyed carrier signal i~ transmitted through the rails from one end of the railroad traek seetion to reeeiver apparatus at the other end whieh ineludes a first channel verifying that both side band frequencies are received at the correct modulation rate, a second channel verifying that at least a minimum signal level of modulated carrier is received, and logic registry means to indicate an unoccupied track ~1499i8 section condition only when the existence of the corresponding distinctive conditions are verified by both channels over a predetermined time period.
Other objects, features, and advantages of the invention will become apparent from the following specification and appended claims when taken in connection with the accompanying drawing.
Sl~MMARY OF THE INVENTIOl~ .
According to the invention, track circuit transmitter apparatus, connected to the rails at one end of a railroad track section, generates and transmits through the rails an alternating current signal having a basic or central carrier frequency which may be in the audio range. This central fre-guency i8 modulated by the FSK method so that the actual track signal has two ~ide band frequencies which deviate from the central carrier in opposite directions by the same amount and which alternate at the selected modulation rate. At the other end of the section, track circuit receiver apparatus is connected to the rails to receive the track signal and energize a track relay only when the received signal has frequency and modulation characteristics corresponding to the transmitted signal, has a signal level above a threshold predetermined in accordance with the transmitted signal level and track section attenuation characteristics, and has a duration in excess o a predetermined ~afe delay period.
In the specific showing, the track circuit transmitter comprises a relatively conventional combination of elements _ 4 --to generate an FSK signal which is filtered and amplified for transmission through the track section rails. It provides a frequency modulated signal with two side fre-quencies spaced about a central carrier frequency and deviating an equal amount on both sides at a selected modulation rate. The receiver apparatus includes two veri-fication channels or circuit networks, each verifying the presence, in the received signal, of predetermined charac-teristics of the transmitted signal. Each of these networks is fed from the rails through a common input network including two filters and an amplifier coupled in series. The filters are both of the band-pass type, the first passing a frequency band centered about the modulated central frequency, and the second passing the two side freguencies of the FSK signal.
The filtered signal is amplified to a sufficient operating level for verification. The fir~t verification network receives the filtered signal with the side band frequencies and demodulates thi~ signal to determine the presence of both frequencies at the proper spacing from the central carrier frequency. The demodulated signal is then decoded to as~ure that the modulation rate of the received signal is the same as that of the transmitted signal. If these condi-tions are met, the first network supplie~ an output signal to one input of a logic AND element. The second verification channel or network receives the filtered and amplified signal from the input network and, by a conventional linear amplifier, determines whether the received signal has at leat a minimum ,:
~1499~8 level, that is, equals or exceeds a preset threshold level of this amplifier. The selected threshold level is determined by subtracting the predetermined attenuation caused by the characteristics of the track rail circuit from the known signal level transmitted from the other end of the section.
If this threshold is at least equaled, an output from the linear amplifier is supplied to the second input of the logic AND element. If both verification or concurrence signals are present, the output of the AND gate is supplied through a delay circuit to energize the track relay. This delay element requires that the output of the AND gate be of at least a pre-determined duration to eliminate very short, extraneous noise signals induced into the track circuit which might meet the other verification tests. The track relay is energized then only if the received signal is verified as having the proper characteristics, that i8, an unoccupied track ~ection indica-tion i8 registered only by the reception of a correct signal.
DESCRIPTION OF THE PREFER~ED EMBODIMENT
Prior to defining the invention in the appended claims, I shall describe a preferred embodiment of the invention as illu~trated in the accompanying drawing which shows schemati-cally, and principally by block diagram, the transmitter and receiver apparatus for a frequency modulated railroad track circuit embodying the invention.
Referring now to the drawing, across the top are conven-tionally shown the two rails 1 and 2 of a section T of a stretch of railroad track. Connected to the rails at the ~9918 right end of the section is a track circuit transmitter, diagrammatically shown below the horizontal dashed line separating the upper and lower parts of the drawing. Con-nected to the rails at the left end of the section is the track circuit receiver which is similarly illustrated above the dashed line. The rails of the section T may or may not be insulated from the adjoining rails of the stretch by insulated joints although none are here shown. The track circuit of the invention is preferably used in an overlay fashion without insulated joints, e. g., for a highway crossing warning installation. However, as is well known in such track circuits, the receiver apparatus is tuned to respond only to the signal from the corresponding track circuit transmitter so that no interference with other track clrcuits occurs, for example, such as used for traffic control signaling.
The transmitter include9 a modulation signal or code generator 6 which produces the modulation signal having a frequency Fm which may be on the order of 7.5 to 20.0 Hz.
Any known arrangement, preferably using solid-state elements, may be used to produce this code or modulation frequency signal. A signal generator and modulator unit 8 produces a signal, having a central carrier frequency Fc, which is then modulated, preferably by the FSK method previously discussed.
Generator 8 is driven by the signal from generator 6 to produce the FSK modulated output which is alternately, at rate Fm~ of frequencies Fl and F2. Preferably, frequencies 1~49918 Fl and F2 deviate from the basic or central frequency Fc by the same amount, that is, by a frequency shift on the order e4r~d A f, for example, 25 Hz above the below, respectively, the central frequency. If the central frequency Fc is selected to be within the audio range, then the side band frequencies Fl and F2 are also audio frequencies.
m e output of generator 8 is passed through a band pass filter 10 and then, for safety, shaped, e. g., squared, and limited in the shaper and limiter unit 11 to provide a sub-stantially constant amplitude signal as the supply voltagefor the generator varies. This squared wave form is then applied to pre-amplifier 12 and power amplifier 13, which are coupled by interstage transformer 14, to furnish sufficient signal level for transmission through the rails, to which the amplifiers are coupled by output transformer 16. A final series LC band pas~ filter 17, tuned to the central frequency Fc~ iB inserted in the rail connections, to block interfering signals in the rails from the transmitter network. The trans-mitter arrangement thus applies to the rails 1 and 2 of the track circuit a signal characterized by having two side fre-quencies Fl and F2, which alternate in time at the modulation frequency Fm as driven by the modulation generator 6. This signal has substantially a constant amplitude, even though the supply voltage may vary, to permit a stability in the track circuit operation, that is, in the shunt condition which it will detect.
The track circuit receiver apparatus illustrated above the dashed divider line receives the track signal from the rails at the other end of section T through a band pass filter 19 which, for example, may be a series LC circuit tuned to a narrow frequency band, centered on the base fre-quency Fc but wide enough to include side frequencies Fl and F2. A second band pass filter 20 coupled by input transformer 21 is more selective than the initial filter 19, being tuned to pass the side frequencies Fl and F2. The output of filter 20 is coupled by the impedance amplifier unit 22 to a first and second verification circuit network outlined by the dashed blocks 23 and 24, respectively.
The first verification network includes a signal shaper 26, a demodulator 27, and a decoder element 28. Unit 26 shapes the signal received from filter 20 through the impe-dance amplifier into a square-wave form of constant amplitude and supplies this output to the demodulator 27. This latter unit demodulates the received signal only if the frequency deviation i6 identical to the predetermined shift in the transmitter apparatus, for example, + 25 Hz from the central carrier frequency Fc. If this condition exists, the output signal at the demodulation rate is applied to decoder unit 28, which inc1ude-q a resonant LC circuit. Unit 28 generates an output, that is, a concurrence or true signal, only if the frequency or code rate of the demodulated signal coincides with the modulation frequency or code rate Fm of the signal supplied by the transmitter apparatus. Thus, the first ~149918 verification network 23 produces a true or concurrence output signal only when the input from band pass filter 20 is an FSK signal, including frequencies Fl and F2, alternating at the modulation frequency Fm and based on, and with equal deviation from the central carrier frequency Fc as transmitted from the other end of the track circuit by the transmitter apparatus. In other words, a true signal output by network 23 designates that the received rail signal has frequency and dulation characteristics identical to the signal transmitted into the rails at the other end of the section.
The second verification circuit network 24 consists solely of a linear amplifier 32 with a preselected threshold level. Amplifier 32 produces an output signal only when the input received from band pass filter 20 through amplifier 22 equals or exceeds this threshold. This level is preselected in accordance with the calculated or known attenuation of the tran~mitted signal by the associated track circuit which, in this case, includes rails 1 and 2 of section T. Thus, an output signal from this amplifier signifies that the track signal received i8 from the transmitter apparatus at the other end because it has at least the expected minimum level.
The true or concurrence signal outputs of networks 23 and 24, specifically from decoder 28 and amplifier 32, re~pec-tively, are applied to the ~wo inputs of the logic D gate 29. As is conventional, element 29 produces an output signal only when both input signals are present. An output from element 29 is applied to a delay circuit 30 which has a ~1499~-8 predetermined delay time, for example, on the order of two to three seconds. Unit 30 produces sufficient energy to energize track relay TR but only when the input signal from AND gate 29 is of a longer duration than this predetermined delay time.
When energized, relay TR picks up, closing its front contact to indicate an unoccupied condition for section T. However, if relay TR becomes de-energized, for example, when the AND
element output ceases, it releases immediately to register an occupied track section condition. The delay time of unit 3~0 assures that relay TR does not pick up in response to input pulses of very short duration from an interference source which may, however, have frequency and amplitude character-istics equivalent to proper track signal. In other words, this delay time inhibits the response of relay TR to a variable noise source which might activate simultaneous re~ponse, for a very short interval, by verification networks 23 and 24.
The receiver apparatus, therefore, energizes track relay TR, to register an unoccupied track circuit condition, only when the signals received through the track circuit rails from the tran~mitter apparatus have the correct or true characteristics. These characteristics include being an FSK
modulated signal with the same side frequencies Fl and F2, deviating from a central carrier frequency Fc by the same frequency shift amount, having the same dulation frequency Fm as transmitted, an amplitude equal to or greater than a predetermined threshold level selected in accordance with the ~4~918 the known track circuit attenuation characteristics, and a ~-duration greater than a predetermined safety time period.
The satisfying of these conditions, without which track relay TR is not energized, renders the receiver apparatus very selective with regard to the signals received, thereby increasing its degree of nonresponse to interferin~ or noise signals present in the rails 1 and 2 of the track circuit.
Although I have herein shown and described but a single preferred form of a frequency modulated track circuit arrange-ment embodying the invention, it is to be understood thatvarious changes and modifications within the scope of the appended claims may be made without departing from the spirit and scope of my invention.